In order to form a microwave-assisted recording head, it would be important to design and fabricate a spin-torque oscillator that can oscillate stably at a low drive current and generate an in-plane high-frequency magnetic field sufficient to resonate the magnetization of a magnetic recording medium.
The maximum density of a current that can be passed through the spin-torque oscillator is 2×108 A/cm2 when an element size is about 70 nm, for example. At a current density higher than or equal to 2×108 A/cm2, the spin torque oscillator may be compromised by heat generation and migration. For this reason, it is important to design a spin-torque oscillator that can oscillate at the lowest possible current density.
Conventionally, a spin injection layer is formed of a ferromagnet such as a Co/Ni artificial lattice. For example, in an element having a size of 40 nm by 40 nm, theoretically without interaction between the element and a magnetic pole, an oscillation layer of about 25 nmT can be made to start oscillating at about 0.5×108 A/cm2. However, in an actual magnetic head, since the magnetization of the oscillation layer interacts with the magnetization of an auxiliary magnetic pole close to the oscillation layer in a magnetostatic manner, a current density necessary for the oscillation may become undesirably large.